Number 292, October 23, 1996 by Phillip F. Schewe and Ben Stein
CAN PULSATING STARS EMIT GRAVITATIONAL WAVES detectable on Earth? Gravitational
wave monitors are essentially elaborate strain gauges employing suspended
masses or interferometers that signal the appearance of faint, passing
distortions in space. In assessing the chances of observing such a wave,
theorists have largely concentrated on the hypothetical disturbances caused
by binary systems, such as pairs of black holes or neutron stars. Now Nils
Andersson of Washington University (nils@howdy.wustl.edu) and Kostas Kokkotas
of the Aristotle University of Thessaloniki (Greece) report on the likelihood
of measuring gravitational waves coming from single stars, such as pulsating
neutron stars created in a supernova. Their calculations, which take into
account details of general relativity overlooked in many previous studies,
show, for example, that the gravitational bang from Supernova 1987A could
have been seen by present-day detectors if as little as one millionth of
a solar mass of the supernova's energy had been dispatched in the form
of gravitational waves. If this fraction were as high as several percent,
the study shows, neutron stars formed in supernovas as far away as the
Virgo Cluster could be detected gravitationally at a rate of several per
year. The authors also show how the gravitational wave signal can be decoded
to provide detailed information about the star. (Physical Review Letters,
11 November 1996.)
WAVEPACKET TECHNOLOGY . Femtosecond laser pulses have a truncated spatial
extent, and according to classical wave mechanics they must be represented
not as a wave at a single wavelength but instead as a superposition of
waves at different wavelengths (or colors). When such a pulse is absorbed
by an atom or molecule, the resultant quantum state is also a superposition
of different energy quantum levels. Putting atoms or molecules into such
"wavepacket" states can be exploited for practical ends. Physicists
at the Weizmann Institute in Israel and the National Research Council of
Canada (Albert Stolow, NRC, 613-993-7388) have separated isotopes of bromine
molecules in a gas by using femtosecond laser pulses to create wavepacket
states. The evolution of these packets is different for the two isotopes,
and if a second laser pulse is applied at just the right moment, one isotope
can be ionized (and extracted) while the other isotope remains behind.
The researchers believe that wavepacket techniques can be used to control
chemical reactions and that creating wavepackets in semiconductor materials
can potentially lead to the development of ultrafast terahertz switches,
a thousand times quicker than the fastest existing switches. (I.Sh. Averbukh
et al., Phys Rev Lett, 21 Oct; see also /physnews/preview)
HELIUM DROPLETS AS A NANOSCALE CRYOSTAT . The study of tiny clusters
of metal atoms (clusters represent a form of matter poised between the
atomic and bulk worlds) is complicated by surrounding factors. If placed
on a solid substrate, the clusters (most of whose atoms are at or near
a surface) take on some of the properties of their support medium. By contrast,
clusters in a beam are not contaminated by a surface but exist in a relatively
hot environment. Scientists at the Max Planck Institute in Gottingen (Germany)
get around these problems by embedding clusters (of silver, indium, and
europium) in droplets of helium. The clusters (with as few as 100 atoms)
are stable and at a precisely defined temperature of 0.37 K, making possible
high resolution spectroscopy and perhaps the study of superconductivity
in cold metal clusters. (A. Bartelt et al., Phys Rev Lett, 21 Oct.)
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